Washington (CNN) -- The president of South Korea has vowed "resolute" measures against North Korea for its alleged attack on a South Korean warship, South Korea's Yonhap News Agency reported Thursday.

A five-country committee announced Thursday morning in Seoul that they had concluded a North Korean submarine fired a torpedo that sunk the South Korea warship in March.

Investigators recovered a propeller from the torpedo that blew the Cheonan in half on March 26, Yoon Duk-yong, the committee's co-chair, told reporters.

The propeller was from the kind of "torpedoes that were exported from North Korea and the letters and the fonts on the torpedo are the are the same that are used by North Korea," Yoon said at a Thursday morning news conference. "This torpedo was manufactured in North Korea."

Yoon said that the investigation found that a small to mid-sized North Korea sub "fired the torpedo that sunk the Cheonan vessel and retreated back to their border."

As the 1,200-ton vessel went down, 46 sailors were lost near disputed waters in the Yellow Sea.

"(We) will take resolute countermeasures against North Korea and make it admit its wrongdoings through strong international cooperation and return to the international community as a responsible member," President Lee Myung-bak told Australian Prime Minister Kevin Rudd in phone talks, according to Lee's office, Yonhap reported.

North Korea denied torpedoing the warship just as the team of investigators in Seoul unveiled their report.

"We had already warned the South Korean group of traitors not to make reckless remarks concerning the sinking of warship Cheonan of the puppet navy," North Korea's National Defence Commission said in a statement, according to the Korean Central News Agency. "Nevertheless, the group of traitors had far-fetchedly tried to link the case with us without offering any material evidence."

"It finally announced the results of the joint investigation based on a sheer fabrication" the defense commission said, according to the state-run KCNA. The commission called the new report part of a "smear campaign."

The White House backed the report issued Thursday in Seoul, saying it "points overwhelmingly to the conclusion that North Korea was responsible for the attack."

"This act of aggression is one more instance of North Korea's unacceptable behavior and defiance of international law," said a statement by White House Press Secretary Robert Gibbs. "This attack constitutes a challenge to international peace and security and is a violation of the Armistice Agreement."

President Obama spoke with South Korean President Lee bak on Monday and "made clear that the United States fully supports the Republic of Korea, both in the effort to secure justice for the 46 service members killed in this attack and in its defense against further acts of aggression," Gibbs said.

"North Korea must understand that belligerence towards its neighbors and defiance of the international community are signs of weakness, not strength," Gibbs' statement said. "Such unacceptable behavior only deepens North Korea's isolation. It reinforces the resolve of its neighbors to intensify their cooperation to safeguard peace and stability in the region against all provocations."

The United States has a mutual defense treaty with South Korea and Japan to defend "against any aggression," so if a military confrontation develops, the United States would be responsible for defending South Korea, a U.S. military official said.

"I don't think it will come to that," the official said. "They know they need to have a response, but there is too much at stake for South Korea to have a confrontation on the Korean peninsula. North Korea has nothing to lose, but South Korea is a serious country with a huge economy."

There are military options for South Korea beyond firing missiles, said John Delury, who studies North and South Korea at the Asia Society.

Anything combative would hurt South Korea economically, Delury said, but the country could increase its naval presence along the line that divides South and North Korea in the waters surrounding the countries. He notes that comes with a risk.

"Those actions could trigger a conflict," he noted.

U.S. Secretary of State Hillary Clinton, who will visit Seoul next week, will talk with the South Korean government about the investigation, Assistant Secretary Campbell said.

Clinton will also visit Japan and China during her trip, and the North Korean issue is likely to be high on the agenda.

Clinton will have "the closest possible consultations with Japan, China and South Korea about the next phase," Campbell said.

On Monday, President Obama spoke on the phone about the investigation with President Lee.

The president reiterated "the strong and unwavering commitment of the United States to the defense and the well-being of its close friend and ally, the Republic of Korea," a White House statement said about the conversation.